The present invention relates to oximeters, and in particular to techniques for ambient light cancellation in pulse oximeters.
Pulse oximetry is typically used to measure various blood flow characteristics including, but not limited to, the blood-oxygen saturation of hemoglobin in arterial blood, the volume of individual blood pulsations supplying the tissue, and the rate of blood pulsations corresponding to each heartbeat of a patient. Measurement of these characteristics has been accomplished by use of a non-invasive sensor which scatters light through a portion of the patient's tissue where blood perfuses the tissue, and photoelectrically senses the absorption of light in such tissue. The amount of light absorbed is then used to calculate the amount of blood constituent being measured.
The light scattered through the tissue is selected to be of one or more wavelengths that are absorbed by the blood in an amount representative of the amount of the blood constituent present in the blood. The amount of transmitted light scattered through the tissue will vary in accordance with the changing amount of blood constituent in the tissue and the related light absorption. For measuring blood oxygen level, such sensors have typically been provided with a light source that is adapted to generate light of at least two different wavelengths, and with photodetectors sensitive to both of those wavelengths, in accordance with known techniques for measuring blood oxygen saturation.
Known non-invasive sensors include devices that are secured to a portion of the body, such as a finger, an ear or the scalp. In animals and humans, the tissue of these body portions is perfused with blood and the tissue surface is readily accessible to the sensor.
One problem with oximeter measurements is that in addition to receiving the light that was directed at the tissue, ambient light is also detected by the photodetector. Attempts can be made to block out ambient light, but some amount of ambient light will typically be detected. One particular concern is the light at the power line frequency of fluorescent or other lights, which is 60 Hz in the United States and 50 Hz in Europe and other countries.
Since a single photodetector is typically used, the light of different wavelengths, such as red and infrared, is time multiplexed. The detected signal must be demultiplexed. The demultiplexing frequency must be high enough so that it is much larger than the pulse rate. However, choosing a demultiplexing frequency is also impacted by the ambient light interference. One issue is the aliasing of harmonics of the AC power line frequency. U.S. Pat. No. 5,713,355 discusses a technique of altering the demultiplexing frequency depending upon the amount of ambient interference detected at each frequency.
U.S. Pat. No. 5,885,213 discusses subtracting a dark signal (detected ambient light) from the detected light signal. This is accomplished by leaving both the red and infrared light emitters off, in between turning them on, so that a “dark” signal supposedly composed of the ambient light present can be detected. This can then be subtracted from the desired signal. Other examples of patents dealing with the ambient light issue are U.S. Pat. No. 6,385,471, U.S. Pat. No. 5,846,190 and U.S. Pat. No. 4,781,195.
U.S. Pat. No. 6,449,501 discusses using a notch filter to filter out line frequency. However, the sampling rate is described as being set to twice the fundamental frequency of the power line interference, leaving higher harmonics of the power line interference as a problem, and it is unclear how the interference can be filtered without filtering the modulation frequency. Another example of a notch filter being used is set forth in U.S. Pat. No. 4,802,486, which uses a notch filter for the EKG signal.